Thermal Design Concepts

Hi. I'm Mark Davis Marsh. And this is Thermal Design. We're going to be talking about thermal design concepts and just some of the terminology behind thermal design. This is part of switching power supply design considerations.
Why does thermal performance matter? I think everyone heard about the lithium batteries that caught on fire either on airplanes or on laptops. And it's usually a problem with the batteries getting too hot.
And the same kind of problem can happen with other types of components, with ICs. When they get too hot, their reliability starts to go down. And you can get problems.
So what is the way that we analyze this? So there was a chemist named Arrhenius in the early 1900s. And he came up with an equation that basically looks at an acceleration factor.
An acceleration factor is basically how long it takes for a chemical reaction to happen. And what you see is that it's based on an activation energy and a Boltzman constant and that it's also determined by the temperature. And if we look at some common activation energies for things that happen in electronic circuits like electro-migration or silicon defects, these tend to happen in the range of 0.3 to 0.9 electron volts. If we plug that into the equation and then we just do a sweep of the acceleration factor versus the junction temperature of our silicon, we can see that in general for every 10 degree rise in temperature, we are twice as likely to have one of these bad defects or electro-migration events. And so what this translates to is for every 10 degrees hotter your device is, you are diminishing the lifetime of your device by a factor of two.
So what causes the high temperature? Of course, you can have high ambient temperatures. But the temperature will also increase due to power losses on your design. So the temperature coefficient of copper increases at hotter temperatures so that you can see a 39% increase in resistance over 100 degree rise. This is going to increase the losses of your design.
So there's examples here of how that would affect your inductor and also your traces. But there's other loss mechanisms that get worse at high temperatures. You have higher leakage currents in your diodes. And you have switching time transitions that increase. And these are going to lower the efficiency of your power supply and generally cause an increase in temperature rise.
So if we look at why thermal performance matters, basically good thermal performance is going to allow more output power. If we design the part so that it is kept cold, then we'll get better efficiency. We'll get longer lifetime. And we'll actually be able to raise the output power of a design versus a design that's poorly thermally designed.
Solution size is directly related to the thermal performance. So if a design is very small but has poor thermal performance, we won't be able to use that design. So you can't have a super small design that doesn't look at the thermal performance. Also, ambient temperatures, if you need a design that goes up to very high ambient temperatures, you're going to need to look at how you are cooling your devices so that they don't exceed the maximum junction temperatures that they're rated for. And of course, like we said, this is going to improve performance and increase reliability.
So how are the different ways that we can get heat out of our silicon, out of our power supply design, and into the ambient air, which is generally how we're doing the heat transfer? So an IC power loss is directly translated to heat in the device. And there's three basic methods that you can remove heat from a design.
There's conduction, which is basically a transfer of energy between objects that are in physical contact. So if you have a piece of copper directly touching another piece of copper and this one is hot, then you will get heat flow through the two copper conductors that are touching. And this is conduction.
Convection is actually another form of conduction. But it just happens to be when the object, the secondary object, is a fluid, such as air. So in convection, you have air flowing over a solid surface. And that causes convective heat transfer.
The final method of heat transfer is radiation. Now with radiation, you are basically getting emission and absorption of electromagnetic radiation. This is a secondary path for heat leaving the PCB and the IC package and really only needs to be looked at for cases when you have a really tight design and you need to look at every single aspect of getting heat out of your package. Conduction and convection are your primary methods for removing heat from the design.
So when we look at thermal characterization, we usually end up creating an equation such as the one you see here. And what you can see is that this equation gives you a theta. And that basically is just a thermal resistance or impedance. It then has two subscripts, j and x.
So the first subscript j denotes the junction of your semiconductor IC. The second subscript x denotes whichever environment you are transferring heat to. So if it's theta JA, this is a transfer of heat from the junction to the ambient environment, A. If it's theta JC, this is a transfer of heat from the junction to the case, C. So that's how you look at this parameter.
And then you go to the other side of the equation. And you get a temperature difference. So this is TJ-- so that's the temperature of the junction-- minus TX. So that's the temperature of whatever your heat is transferring to.
So TJ would be T junction. TA is T ambient. TC is case temperature, et cetera.
And then that's divided by the power that you're trying to dissipate in watts. So if you have 5 watts of power dissipation, that's where this would go into the equation. And basically, what you have created is an analogy of thermal heat flow that looks very similar to an equation for resistance.
So if you look at the analogy, you can see that for an electrical resistor, the resistance is equal to a change in voltage divided by the current. If we look at our thermal analogy that looks very similar to this, you can see that the thermal resistance, or theta, is equal to a temperature difference divided by the power. And we can see that we can basically model our whole thermal heat path as a resistor. And then we can use this to calculate thermal resistances. And this will allow us to estimate junction temperatures.
Now I showed you the theta type. There's also a xi type parameter. What's a difference? A theta type thermal impedance basically is concluding that all of the heat flows from one location to another. Basically, it's isothermal. And there is no heat transfer in any other path.
This is not typical of a normal design. As you have heat paths in multiple directions. But it's the easiest type of parameter to measure. So it's commonly seen on data sheets.
You also have a xi type. Xi basically is a parameter where the heat flow is measured from one place to another, but it is not excluding the possibility that heat can flow in a variety of different other paths. So it's non-isothermal. And this is more typical of a actual power supply design. Unfortunately, it's very hard to get this design as it's not on typical data sheets.
So let's look at what measurements usually are typically on data sheets. Typically, you can get a R theta JA, which is basically the thermal impedance from the junction to the ambient. This will be measured using a JEDEC standard.
You can also get an R theta JC, so then thermal impedance from the junction to the case. Now the junction to the case is kind of misleading. For packages that don't have an exposed DAP, the case is considered the top of the package. However, on a lot of data sheets with the newer power parts, the exposed pad or the DAP of the device is considered the case. So you need to be careful when you're looking at the devices whether you're looking at a R theta JC for the top of the case or you're looking for a R theta JC that's actually the exposed pad of the device.
So if we have a R theta JC and a R theta JA, can we use these to develop a resistive circuit so that we can measure the ambient air temperature, know our power dissipation, and then estimate the junction temperature? And the answer is that we can and that it works fairly well. So if you look at the resistor model that I have on the top right, you can see that the potential is from the junction of the device to the ambient air. And there are two different paths from the junction to the ambient air.
There's the path that goes from the top of the case and then has a case-- the case top will then convect as it touches the air. And you'll get convection from the top of the case to the ambient air. And that goes to-- you have that path.
You also have the bottom path where the heat goes through the bottom case or the exposed pad. Now this exposed pad resistance will be much smaller than the resistance to the top of the case. And so this is usually your primary heat path.
Now once you get to the bottom of that case, you still need to get from the case to the ambient air. And usually, the method of heat transfer is through your PC board design. So your effective theta JA is highly dependent on how well you design the PC board to get heat out of the bottom of the case of the component and to the ambient air.
And what we can do is we can actually make a circuits 101 diagram of all of these different electrical resistances. And you can see that I've put some typical values here for the value of the thermal resistance of 1 ounce copper, the thermal resistance of [INAUDIBLE] and the thermal resistance from a surface of your PC board to the air. So that's the main one.
Your board has limited surface area to convect heat off of the board. And since this is usually your primary heat path to remove heat out of the design, you're going to need to look at the ability of the surface area of the board to convect heat off. This is that theta SA number on the bottom.
And you're going to want to make your board big enough to get rid of the heat. Now if you can't make the board big enough to do that, this is where you would add a heat sink or another device like that, which has more surface area than your board. And that extra surface area allows it to have a lower resistance to getting the heat off of the board and into the ambient air.
Now if you look at different data sheets, it's pretty hard to see what kind of information you need. And the problem is different chip companies will put different parameters onto their devices. They can't decide whether they want theta JA. Or if they do use theta JA, they will not use the JEDEC standard. And you'll get different values.
But basically, what you want to look at in the data sheets is you want to get a low theta JC number if the theta JC is listed. And that means the chip is very easily able to get heat out of the package. Now theta JA you can use as a reference. But it is very specific to the PC board that was used. So it may not be appropriate for your design.
So how to compare. Look for theta JC. It's the best measurement of the package thermal performance. It's not board-dependent.
And so theta JA, which is board-dependent, is going to be a function of the PCB. And it's really going to depend on how well you do the thermal design. You're also going to want to look at efficiency. If the part has higher efficiency, there's going to be less power dissipation and so thus, less heat to get out of your design.
And if you want to look at theta JA, make sure that it is a comparison of the same boards. So make sure that they're following the JEDEC standards and you're comparing boards of similar size. So this has been Thermal Design Parameters. Thank you for listening.

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Date:
April 29, 2013

These videos will cover switching power supply design considerations.

This series includes:

Thermal Design Guidelines and Recommendations: Strategies and recommendations for maximizing the thermal performance of your designs.

Thermal Design Concepts: Intro to various factors that impact thermal performance and why they're important to consider in designs.